A B767-300 ROLLED ONTO THE TOWBAR AFTER PUSHBACK WHEN THE GROUND CREW REPORTEDLY DISCONNECTED THE BAR WITHOUT COORDINATING WITH THE CAPTAIN TO HAVE BRAKES SET.

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I WAS FLYING AS FO. THE WX WAS VFR DAY; AND WE BEGAN THE PUSH ON TIME. THE CAPT CLRED ME TO START BOTH ENGS. THE PUSHBACK STOPPED AS I WAS STARTING THE SECOND ENG; AND THEN WE BEGAN TO MOVE FORWARD IN WHAT I PRESUMED TO BE A TOW FORWARD. ALMOST IMMEDIATELY; I HEARD THE CAPT SAY TO THE TUG DRIVER 'I CANNOT SET THE BRAKES UNTIL YOU STOP' AND THE ACFT CAME TO AN ABRUPT STOP. I ASSUMED THIS TO BE THE TUG DRIVER SLAMMING ON HIS BRAKES. AS I COMPLETED THE START PROC; I WAS AWARE OF FURTHER COMS BTWN THE CAPT AND THE TUG DRIVER; AND UPON COMPLETION OF START; THE CAPT INFORMED ME THAT WE 'HAD COME UNHOOKED; AND THE AIRPLANE HAD ROLLED ON TOP OF THE PUSH BAR.' WE WERE ADVISED BY A MECH ON THE GND THAT WE WOULD NEED TO BE TOWED BACK IN TO THE GATE FOR A GEAR INSPECTION PRIOR TO DEP. THE CAPT ASKED FOR THE JETWAY; AND PARTICIPATED IN THE INSPECTION OF THE GEAR WITH MAINT. UPON HIS RETURN TO THE COCKPIT; HE INFORMED ME THAT THE TUG DRIVER WAS TELLING OTHER PERSONNEL THAT HE HAD GIVEN THE CAPT THE 'SET BRAKES' COMMAND AND THAT THE CAPT HAD RESPONDED 'BRAKES SET; PRESSURE NORMAL' AT WHICH POINT HE COMMANDED THE DISCONNECT OF THE TOW BAR AND THE PLANE THEN ROLLED FORWARD. ALTHOUGH I WAS NOT INVOLVED IN THE COM OF THE PUSHBACK; AND COULD NOT HEAR THE TUG DRIVER; I CAN ASSURE YOU THAT THE CAPT NEVER MADE THE 'BRAKES SET' SOP CALL PRIOR TO THE ACFT ROLLING FORWARD AND UP ON TO THE TOW BAR. MAINT WROTE UP THE INCIDENT; INSPECTED THE GEAR; AND RELEASED THE ACFT. WE ALSO DEBRIEFED WITH THE STATION MGR IN THE COCKPIT; AND THEN DEPARTED APPROX 1 HR LATE. THE FLT CONTINUED AND WE ARRIVED WITHOUT FURTHER INCIDENT.